The present invention relates to a brake master cylinder for motor vehicles and more particularly to a split-type master cylinder in which dual brake fluid pressure is applied independently of each other to sets of wheel brake cylinders.
A brake master cylinder requires, as is well known, a relatively large volume of brake fluid at the initial stage of brake application and relatively high pressure at the subsequent stage thereof. For instance, in the brake master cylinder associated with disc brakes, the relatively large volume of brake fluid is necessary to bring the friction pads of the disc brakes to abutment to the rotor while the relatively high brake pressure is necessary to permit the friction pads to squeeze the rotor.
In order to satisfy such requirement of the brake master cylinder, it has been proposed to prepare a stepped bore of the cylinder in which a stepped piston is slidably fitted to define a fluid chamber between the large diameter portion of the stepped piston and the small diameter portion thereof so as to supply brake fluid to the wheel brake cylinders only at the initial stage of brake application.
According to the conventional split-type brake master cylinder, however, a compensation port for a primary pressure chamber is fluidically connected also to the fluid chamber described above. More specifically, it is a conventional practice to connect the compensation port for the primary pressure chamber formed between a primary piston and a secondary piston to the above-mentioned fluid chamber, thereby to permit normal fluidic communication of the fluid chamber with a brake fluid reservoir.
Therefore, the pressure in the fluid chamber is subject to variation in response to depression speed of brake pedal because of the compensation port with the results that the master cylinder pressure varies with the speed of depression the brake pedal.